The Collaborative International Dictionary of English v.0.48:

Scientific \Sci`en*tif"ic\, a. [F. scientifique; L. scientia
   science + facere to make.]
   1. Of or pertaining to science; used in science; as,
      scientific principles; scientific apparatus; scientific
      observations.
      [1913 Webster]

   2. Agreeing with, or depending on, the rules or principles of
      science; as, a scientific classification; a scientific
      arrangement of fossils.
      [1913 Webster]

   3. Having a knowledge of science, or of a science; evincing
      science or systematic knowledge; as, a scientific chemist;
      a scientific reasoner; a scientific argument.
      [1913 Webster]

            Bossuet is as scientific in the structure of his
            sentences.                            --Landor.
      [1913 Webster]

   Scientific method, the method employed in exact science and
      consisting of: (a) Careful and abundant observation and
      experiment. (b) generalization of the results into
      formulated "Laws" and statements.
      [1913 Webster]

WordNet (r) 3.0 (2006):

scientific
    adj 1: of or relating to the practice of science; "scientific
           journals"
    2: conforming with the principles or methods used in science; "a
       scientific approach" [ant: unscientific]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "scientific":
   close, constant, delicate, detailed, direct, disciplinary,
   down-to-earth, earthy, even, exact, express, faithful, fine,
   hardheaded, inerrable, inerrant, infallible, mathematical,
   matter-of-fact, methodical, meticulous, micrometrically precise,
   microscopic, nice, orderly, painstaking, pinpoint, positivistic,
   practical, practical-minded, pragmatic, precise, rational, realist,
   realistic, reasonable, refined, religious, religiously exact,
   rigid, rigorous, sane, scholarly, scientifically exact,
   scientistic, secular, sensible, severe, sober-minded, sound,
   sound-thinking, square, straight-thinking, strict, subtle,
   systematic, technical, technicological, technological, thorough,
   undeviating, unerring, unideal, unidealistic, unromantic,
   unsentimental, well-ordered, well-regulated, worldly